H.v. testing transformer winding arrangement

ABSTRACT

A high-voltage transformer for producing test voltages has a low-voltage primary winding, a high-voltage secondary winding, and a cascade winding. The primary winding is formed by two separate low-voltage windings wound on separate limbs of a ferromagnetic core and connected in parallel so that their magnetic fluxes assist in the wound core limbs. The high-voltage secondary winding is wound over one only of the low-voltage windings. The cascade winding is formed by two separate lowvoltage windings connected in parallel and in phase, one being wound over the high-voltage winding on one limb and the other being wound over the low-voltage winding on the other limb.

United States Patent Collin [151 3,678,429 [45 uly 18, 1972 [72]Inventor:

gland Fen-anti, Limited, cashire, England [22] Filed: Jan. 8, 1971 [21]App1.-No.: 104,953

[73] Assignee: Hollinwood, Lan- [30] Foreign Application Priority Jan.8, 1970 Great Britain ..901/70 [52] US. Cl ..336/170, 336/184 [51] Int.Cl. 101127/28 [58] FleldoiSearch ..336/l80,l84,170, 145,146,

Frank Anthony James Collin, Oldham, En-

1,761,732 6/1930 Kiichling, ..336/184 X 1,699,292 1/1929 ..-.336/184 X IFOREIGN PATENTS OR APPLICATIONS 1,488,336 5/1969 Germany ..336/ 1841,488,209 4/1969 Germany ..336/184 Primary Examiner-Thomas J KozmaAttorney-Cameron, Kera n & Sutton 1' ABSTRACT A high-voltage transformerfor producing test voltages has a low-voltage primary winding, ahigh-voltage secondary winding, and a cascade winding. The primarywinding is formed by two separate low-voltage windings wound on separatelimbs of a ferromagnetic core and connected in parallel so that theirmagnetic fluxes assist in the wound core limbs. The high-voltagesecondary winding is wound over one only of the low-voltage windings.The cascade winding is formed by two separate low-voltage windingsconnected in parallel and in phase, one being wound over thehigh-voltage winding on one limb and the other being wound over thelow-voltage winding on the other limb.

3Clains,2DrawingF1gures a s a a i a 5 PATENTED JUL 1 8 m2 H.V. TESTINGTRANSFORMER WINDING ARRANGEMENT This invention relates to transformers,and in particular to winding arrangements for transformers used toproduce high voltages for testing purposes.

For the testing of high-voltage electrical equipment it is usual toproduce the necessary voltages by means of high-voltage transformers.The particular type of transformer normally used has one end of thehigh-voltage winding connected to the tank and the other end brought outthrough an insulating bushing. Frequently two or more transformers areconnected in series to produce the desired output voltage, and it isthen necessary to insulate the tanks of all but one of the transformersfrom the ground. In such a case it is no longer possible to supply allthe low-voltage primary windings from a common source because ofinsulation problems, since the tank of the first transformer of theseries will be at earth potential, the

tank of the next transformer willbe at, say, SOOKV, the tank of the nextat lMV and so on. In order to overcome this problem each transformerexcept that at the highest voltage is provided with a low-voltagecascade winding which is used to supply the primary winding of the nexttransformer in sequence.

The usual winding arrangement for a high-voltage testing transformerhaving a cascade winding involves winding the low-voltage primarywinding around one limb of the transformer core. Over this, suitablyinsulated, is wound the highvoltage secondary winding. The end of thiswinding adjacent to the primary winding is connected to the transformertank whilst the high-voltage end of the winding is taken out through abushing. Over the high-voltage winding is wound the lowvoltage cascadewinding, one end of which is brought out so that it may be connected tothe primary winding of a further transformer, and the other end of whichis connected to the high-voltage terminal. The high-voltage terminal ofthe first transformer is connected to the tank of the secondtransformer, which is insulated from the ground against the output ofthe first transformer. The low-voltage primary winding of the secondtransformer is connected between the end of the cascade winding and thetank of the second transformer. If a third is also to be used then thesecond transformer will also have a cascade winding connected in thesame way. The last transformer in the sequence does not require acascade windmg.

Such a winding arrangement has the disadvantage that any capacitive loadwhich may have to be supplied by two or more such transformers connectedin series would be limited owing to the high impedance arising from thereactance between the low-voltage input to the first transformer and thecascade winding of the transformer. This impedance increases as thenumber of series-connected transformers increases.

One method of overcoming this problem is to increase the core size ofthe transformer, but this results in a larger and more expensivetransformer. Another solution which has been employed is to split thelow-voltage primary winding into two parallel-connected windings woundaround separate limbs of the core. The high-voltage winding is woundaround one lowvoltage winding and the cascade winding is wound aroundthe other low-voltage winding. This effects a slight reduction in theimpedance, but is still not entirely satisfactory.

It is an object of the present invention to provide a highvoltagetesting transformer having a cascade winding in which the impedancebetween primary and cascade windings is still further reduced.

According to the present invention there is provided a transformer whichincludes a ferromagnetic core having at least two limbs, a separatelow-voltage primary winding wound around two of said limbs, the twowindings being connected in parallel with one another such that themagnetic flux produced in the wound limbs by one winding assists theflux produced in the wound limbs by the other winding, a highvoltagewinding wound over the low-voltage winding on one only of the woundlimbs, a first cascade winding wound over the high-voltage winding onsaid one limb and a second cascade winding wound over the low-voltagewinding on the other wound limb, the two cascade windings beingconnected in parallel and in phase with one another.

The invention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic sectional view of a winding arrangement inaccordance with the invention, and

FIG. 2 is an equivalent circuit diagram of the winding arrangement ofFIG. 1. I

The core 10 (FIG. 1) of the transformer has two limbs 11 and 12, each ofwhich carries a low-voltage winding 13 and 13 respectively. The twolow-voltage windings are connected in parallel with one another suchthat the magnetic flux produced by one winding in the two wound limbsassists the flux produced by the other winding in the two limbs, andleads are brought out of the tank for connection to the low-voltagesupply.

Over the low-voltage windings 13 on limb 11, is wound the high-voltagewinding 14. The end of this winding adjacent to the low-voltage winding13 is connected to the transformer tank, and the other end of thehighvoltage winding is brought out through an insulating bushing.

The cascade winding is wound in two parts, one of which, 15, is woundover the high-voltage winding 14 on limb ll of the core. The other part15' of the cascade winding is wound over the low-voltage winding 13 onlimb 12 of the core, suitably insulated from the low-voltage winding,since one end of the cascade winding is connected to the high-voltageend of the high-voltage winding 14. The two parts of the cascade windingare connected in parallel and in phase with one another, and the end notconnected to the high-voltage winding is brought out of the tank.

The core of the transformer may have more than two limbs if required,the other limbs not carrying windings.

FIG. 2 shows the equivalent circuit of the transformer described above.Each winding is represented by an impedance given the winding referencefrom FIG. 1. The magnetizing impedance of the primary of a transformerof this type is assumed to be very high and is therefore not shown inthe equivalent circuit. The high voltage output terminal is given thereference H and that of the cascade winding is C.

The two primary winding impedances Z and Z are shown connected inparallel with one another, as is actually the case. Since thehigh-voltage winding 14 is physically wound over one primary winding 13,its impedance 2, is shown in series with the primary impedance 2, Thecascade winding 15 which is wound over the high-voltage winding 14 hasits impedance Z connected to the junction between Z and Z to form a starnetwork, as is usual with three-winding transformer equivalent circuits.The other cascade winding 15' is wound over the primary winding 13', andhence its impedance Z 1 is effectively in series with the impedance ZThe other ends of the two cascade winding impedances Z and Z W are shownconnected together to the terminal C as is the case in practice.

It is the connection of the impedance 2,, which has the effect ofreducing the reactance between the primary and cascade windings of thetransformer as compared with previous winding arrangements using only asingle cascade winding.

What we claim is:

l. A transformer which includes a ferromagnetic core having at least twolimbs, a separate low-voltage primary winding wound around two of saidlimbs, the two windings being connected in parallel with one anothersuch that the magnetic flux produced in the wound limbs by one windingassists the flux produced in the wound limbs by the other winding, ahighvoltage winding wound over the low-voltage winding on one only ofthe wound limbs, a first cascade winding wound over the high-voltagewinding on said one limb and a second cascade winding wound over thelow-voltage winding on the other wound limb, the two cascade windingsbeing connected in parallel and in phase with one another.

2. A high-voltage transformer for producing test voltages comprising aferromagnetic core having at least two limbs, a

primary winding having two separate low voltage windings each wound on aseparate limb of the core, said low voltage windings being connected inparallel so that the magnetic flux produced in the two limbs by onewinding assists the flux produced in the two limbs by the other winding,a high-voltage secondary winding, said high-voltage secondary windingbeing wound over the low voltage winding on one only of the limbs onwhich a low voltage winding is wound and a cascade winding including twoseparate low voltage windings, one of said last mentioned low-voltagewindings being wound over the high-voltage secondary winding, the otherof said last mentioned low voltage windings being wound over thelow-voltage winding on the other limb and said two separate low-voltagewindings forming the cascade winding being connected in parallel and inphase with one another.

3. A transformer as set forth in claim 1 wherein each low voltagewinding forming the primary winding is adjacent the limb, said lowvoltage primary windings and associated secondary and cascade windingsbeing coaxially disposed on said limbs.

1. A transformer which includes a ferromagnetic core having at least twolimbs, a separate low-voltage primary winding wound around two of saidlimbs, the two windings being connected in parallel with one anothersuch that the magnetic flux produced in the wound limbs by one windingassists the flux produced in the wound limbs by the other winding, ahigh-voltage winding wound over the low-voltage winding on one only ofthe wound limbs, a first cascade winding wound over the high-voltagewinding on said one limb and a second cascade winding wound over thelow-voltage winding on the other wound limb, the two cascade windingsbeing connected in parallel and in phase with one another.
 2. Ahigh-voltage transformer for producing test voltages comprising aferromagnetic core having at least two limbs, a primary winding havingtwo separate low voltage windings each wound on a separate limb of thecore, said low voltage windings being connected in parallel so that themagnetic flux produced in the two limbs by one winding assists the fluxproduced in the two limbs by the other winding, a high-voltage secondarywinding, said high-voltage secondary winding being wound over the lowvoltage winding on one only of the limbs on which a low voltage windingis wound and a cascade winding including two separate low voltagewindings, one of said last mentioned low-voltage windings being woundover the high-voltage secondary winding, the other of said lastmentioned low voltage windings being wound over the low-voltage windingon the other limb and said two separate low-voltage windings forming thecascade winding being connected in parallel and in phase with oneanother.
 3. A transformer as set forth in claim 1 wherein each lowvoltage winding forming the primary winding is adjacent the limb, saidlow voltage primary windings and associated secondary and cascadewindings being coaxially disposed on said limbs.